Electro-hydraulic ram control apparatus

ABSTRACT

A control system for the ram of a hydraulic injection molding press, the injection speed or velocity of which is to be precisely controlled according to a predetermined velocity command signal. A programmable relief valve is used to effect accurate control over the flow of hydraulic fluid under pressure to the ram cylinder and is responsive to signals representing diverse conditions of operation of the press. 
     In one application of the concept, velocity and subsequent holding pressure control of a ram are obtained.

This is a continuation of application Ser. No. 620,809, filed Oct. 8,1975, and now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to electro-hydraulic servo controls and methodsand apparatus for controlling the operation of a hydraulic injectionmolding press.

2. Description of the Prior Art

In the development of improved controls for injection molding presses,recent experience, i.e., within the last ten years or so, has includedinvestigation into the possibility of utilizing a computer system toprovide computer programmed servo loop controls for injection pressures,packing pressures and velocity of rams and the like that operated uponthe "next cycle correction" whereby a computer would be used to analyzea cycle to determine if it did not meet the operating conditions asselected and programmed and, if so, a correction to the system would beentered for control of the next cycle. The capabilities of such computeroperated control systems almost dictated from the start that the expenseof the computer itself and of the ancillary equipment would beprohibitive and the slow response of the "next cycle correction" provedto be less desirable than a dynamic operation of a system operated inreal time.

One of the important factors in controlling a hydraulic injectionmolding press is "fill time"; i.e., the time from the moment theinjection starts until the time that the material is packed into thecavities in the mold. This is important because the otpimum conditionfor injecting a plasticized material into a mold is to fill all of thecavities in the mold while the material is still in a molten conditionand will flow freely into each of the cavities. Failure to attain aproper "fill time", which is related to the velocity of the ram, mayresult in marks on the molded part, voids and nitt lines which mayresult in a weakening of the portions of the part to be molded. As onefurther example, the molding of parts to be subsequently plated,dictates that there be no flow lines, voids or nitts because thesedefects will not properly accept the plating material. As a result ofthe introduction of "exotic" molding materials as early as themid-1960's, the "fill time" as determined by the velocity of the rambecame more and more important. In fact, the most important of thefactors which affect the injection cycle was determined to be theprecise control over the "fill time".

However, the material viscosity varies from cycle to cycle dependentupon the nature of the raw material to be plasticized; i.e., the amountof regrind and moisture content. Any change in viscosity will alsoaffect the "fill time". Therefore, injection under a constant pressuretype of system, which was and is used on so-called standard machines,results in a variation in the fill time with changes in materialviscosity. The only way to precisely control "fill time" is to insurethat the conditions that are met as the ram performs its injecting cycleare constantly reproduced during each cycle so that you will always movefrom what is called shot size or "back" position to the forward orfilled position in precisely the same amount of time; e.g., velocitycontrol.

Attempts have been made to precisely control the "fill time" bycontrolling the velocity of the ram for every step of the way. That is,whether you have a constant or a variable velocity, such as a velocityprofile, there must be control over the velocity for every step of theway. One way to accomplish this is to effect a dynamic control under aservo loop-type system which embodies feedback information to force amachine to respond as directed and with great repeatability. Experiencewith servo flow control valves has resulted in a relatively slow speedsystem which is unsuitable because, in some cases, the speed of theresponse of a system exceeds the time of an injection stroke with aresultant undesirable lack of control. Further development led to theuse of a servo flow divider system which provided improved response tocontrol signals but still not fast enough and the flow divider valve isnot readily adaptable to systems having high hydraulic fluid flow rates.One other important limitation of servo type flow controls and flowdividers is a low tolerance for contamination in a hydraulic system sothat a highly efficient filtration system is mandatory and such systemsare prohibitively expensive as well as subject to malfunction. Further,many prior art hydraulic system of diverse nature have included the useof one or more adjustable or programmable pressure relief valves tooperate as such; i.e., one or more different levels of pressure reliefcould be used during different cycles of operation. This may be seen in,for example, U.S. Pat. No. 3,932,083 in which a programmable reliefvalve is applied to an injection molding machine but is operated in anopen loop condition and is not intended to be responsive to a dynamicvelocity error signal to thereby control the pressure of fluid appliedto the piston of a ram in an injection molding machine but only toprovide a sufficiently high pressure to allow a flow control to beoperable. A further prior art example may be seen in U.S. Pat. No.3,721,512 in which a system embodies a pressure control valve as, forexample, a throttle valve, is controlled by a signal representative ofthe melt temperature in an injection molding press.

Prior art apparatus has described a multitude of various and sundry waysof controlling the operation of a hydraulic injection molding press inaccordance with many conditions which are sensed and used in a control.While past systems have been developed for accomplishing the generalpurpose of my invention, the systems known to be in existence have twocommon discrepancies. The first is that expense connected withdeveloping and furnishing the systems has risen as an exponentialfunction of the accuracy desired. Where expense has been a majorconsideration, the systems have lacked any usable degree of accuracy.Heretofore, the conventional method of controlling the velocity of amachine was by means of a hydraulic flow control valve. This means wasnot satisfactory because of slow response. An improved means wasdeveloped using a hydraulic servo valve. Although this improved thespeed of response, it did so at a great increase in expense throughgreater cost of the servo valve and to the high cost of oil filtrationrequired by the valve.

SUMMARY OF THE INVENTION

In carrying out the principles of my invention, one element may be seento be common to all possible uses; that is the application of a pressurecontrol valve that is programmable in accordance with one or more of anumber of signals representative of sensed conditions, to control theapplication of hydraulic fluid under pressure to the hydraulic means forinjecting the material from a conditioner or a transfer pot into themold of an electro-hydraulic injection molding press.

This basic circuit is applicable whether or not the overall apparatus isin one or the other of several modes of operation which may be, forexample, a velocity control of the ram, a condition coinciding with thepressure of the fluid applied to drive the ram or the pressure of thematerial in the mold itself.

In one embodiment of my invention, a vastly improved dynamic velocitycontrol system utilizing a variably controllable pressure relief valveis operated in accordance with an error signal derived from a source ofvelocity command signal which may be variable in nature, and the actualvelocity of the ram as may be determined by differentiating an analogposition signal from a source normally found on such hydraulic injectionmolding presses to provide an unexpected improvement in response time,and precise, real time control over the injection portion of the cycleof such apparatus; without necessitating flow control of hydraulic fluidfrom the pump to the ram cylinder.

I have also discovered that there are additional, heretofore unknown,principles of operation which may readily utilize the concept of thatportion of my invention described above for determining the prior timeto change from one mode of operation to another.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an application of my novel and improved control system to apictorial representation of the essential elements of anelectro-hydraulic injection molding press in which schematic anddiagrammatic representations of the entire system are shown;

FIG. 2 is a similar form of electro-hydraulic injection molding pressshowing a modification in the use of my invention in block schematic anddiagrammatic form; and

FIG. 3 shows representative curves of the events occurring over onetimed cycle of operation of my apparatus.

DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

Referring now to FIGS. 1 and 2 of the drawings, it will be seen that anelectro-hydraulic injection molding press is indicated generally byreference character 10 which includes a source of material 11 that isconnected to a plastic conditioner 12 having a rotating screw and ram 13rotatably disposed within a barrel 14 and terminates in a nozzle 15 atthe left end. A mold 16, having a separable cavity and sprue, is shownin operative engagement with nozzle 15.

A hydraulic motor, shown in the form of a hydraulic cylinder and piston,indicated generally by reference character 18, is shown disposed on theright end of ram 13 and includes a cylinder 19 and an inlet port 20.

A position sensing means is indicated by reference character 21 and isshown adapted to sense the position of ram 13 and includes a stationarypotentiometer winding 22 which may be energized from a suitable sourceof electric current (not shown), and a wiper 23.

In FIG. 1, pressure control valve 24 shown as a relief valve, isconnected to inlet port 20 in hydraulic cylinder 19 through conduit 27which is also connected to a pump (not shown) through a further conduit26. Pressure control valve 24 also includes a pilot relief control unit33 connected to conductor 40, and an outlet 28 which is shown disposedover a tank therefor.

In FIG. 2, a pressure control valve 24, shown as a pressure reducingvalve, is connected to inlet port 20 in hydraulic cylinder 19 throughconduit 27. An inlet port is connected to a pump through conduit 26.Valve 24 also includes a pilot relief control unit 33 connected toconductor 40.

The configurations of the two versions of pressure control valves ofFIGS. 1 and 2 may be recognized as standard symbols customarily used inthe hydraulics industry.

Signal combiner 34 also includes a plurality of input terminals 35, 36,37, 38 and 39 for purposes to be described below.

Referring specifically to FIG. 1, signal combiner 34 is shown havinginput terminal 35 connected to wiper 23 on potentiometer 21 throughconductor 43, velocity detector 41 (which may be a suitabledifferentiation device) and conductor 42. Input terminal 37 on signalcomparator 34 is connected to a movable contact 52 on switch 49 throughconductor 53. Stationary contact 50 on switch 49 is connected to avelocity command means 44 having a knob 45 for adjusting the commandsignal, through conductor 54. Stationary contact 51 is connected topotentiometer winding 23 through conductor 55, velocity characterizer46, having a plurality of input adjusters 47, and conductor 48 andconductor 42.

In FIG. 2 of the drawings, input terminal 36 on signal comparactor 34 isshown connected to a pressure sensor 56 connected to sense the pressurewithin the interior of cylinder 19 and to provide an electrical signalthrough conductor 57 to amplifier 58, through conductor 59 todifferentiating means 60 and to terminal 36 through conductor 61.Terminal 38 on signal combining means 34 is connected to wiper 23 onpotentiometer 21 through conductor 62, velocity detector 63, conductor64, differentiating means 65 and conductor 66.

Referring to FIG. 3, the curves illustrate various conditions sensedduring the operation of the described embodiments.

OPERATION

Referring to FIG. 1, it may be seen that apparatus under considerationprovides operation of the ram on the injection portion of the cycle ofan electro-hydraulic injection molding machine such that the pump orsources of hydraulic fluid under pressure is connected directly to theright end of cylinder 19 and against the piston formed at the right endof ram 13. The output of signal comparator 34 is connected to energizeelectric to hydraulic transducer 33, shown as a pilot valve, inaccordance with the inputs supplied thereto by comparing the velocity ofram 13 with a command signal which may be a constant value as determinedby signal generator 44 or characterized in accordance with the positionof ram 13 by velocity characterizer 46. In any event, the actualvelocity at input terminal 35 is compared with the desired velocity oninput terminal 37 to provide a signal of magnitude which will controlthe operation of pressure relief valve 24 through the transducer 33 sothat the desired velocity is attained.

Referring to FIG. 2, the embodiment illustrated utilizes the same formof operation of a pressure control valve, shown as pressure reducingvalve 24. However, the input terminals 36 and 38 on signal comparator 34receive signals proportional to the rate of change of velocity of ram 13as well as the rate of change of pressure on the hydraulic fluid withinthe right end of cylinder 19. At the appropriate desired magnitude ofthe rate of change of the two signals, the operation of the velocitycontrol system shown in FIG. 1 may be terminated and other appropriatesignals, such as one related to the pressure within the right end ofcylinder 19, a predetermined desired value or a signal representative ofthe pressure of the material within mold cavity and screw 17 may besubstituted.

Curves 67, 68 and 69 in FIG. 3 illustrate the relationship of a typicalsystem employing the principles of my invention wherein curve 67represents a typical velocity having a constant velocity element and afurther curve in dotted form to show a characterized velocity functionand includes a sloped line tangent to the curve as a velocity decreasesas it approaches filling of the mold cavity 17 indicated by dv/dt, therate of change of velocity. Curve 68 illustrates a typical pressurepattern existing within mold cavity 17 which will be seen to beessentially zero until the ram nears the end of its stroke to completelyfill the mold cavity, the pressure continuing to build up as the ram isstopped, as indicated by time base line Z and maintains a fairlyconstant value until it drops off as the material within the mold startsto shrink upon the completion of the cure cycle. Curve 69 represents thehydraulic pressure within the right end of cylinder 19 which, it will beseen, raises from a low value generally as the velocity increases andthen gradually increases to a point near the end of filling of the ramas indicated by time base line Z, at which point it rises rapidly to ahigher value and holds for a time T1, followed by a second interval ofslightly lower value for a timed interval T2 and then gradually returnsto zero. The line tangent to the pressure curve 69 near line Zrepresents the rate of change of pressure and is indicated by dp/dt.

As may now be seen, the operation of FIG. 2 of the illustratedembodiment is directly proportional to the slope of the lines tangent tothe velocity and pressure curves 67 and 69 and may be used to provide aswitch-over for signal combiner-comparator-amplifier 34 to still anothersignal applied to one of its inputs to maintain a pressure applied tothe right end of ram 13 to sufficiently carry out the entire cycle.

I claim:
 1. In combination, a hydraulic injection molding press of theclass having a hydraulically actuated cylinder connected to a ram fortransferring molding material from a chamber into a mold and including asource of pressurized hydraulic fluid having a pressure relief valve andconnected directly to the cylinder, and a controller for controllingoperation of the molding press, the controller comprising(a) acontrollable transducer operating the pressure relief valve to controlhydraulic fluid pressure in the cylinder; (b) ram velocity control meanscomprising means sensing the velocity of the ram as it moves in aninjection stroke and means comparing the sensed velocity with a presetvelocity and producing a deviation signal controlling the pressurerelief valve transducer to thereby control the ram velocity; (c) holdingpressure control means comprising sensing means sensing the hydrauliccylinder pressure, means comparing the sensed pressure with a presetvalue and producing a deviation signal controlling the pressure reliefvalve transducer to control the hydraulic cylinder pressure; and (d)sensing and switching means sensing when a mold has been substantiallyfilled with molding material and responsively switching control of thepressure relief valve transducer from the velocity control means to thepressure control means.
 2. The combination of claim 1 in which thesensing and switching means includes means responsive to mold pressurefor switching control of the pressure relief valve transducer from thevelocity control means to the pressure control means when the moldpressure has increased to a preset value.
 3. In combination, a hydraulicinjection molding press of the class having a hydraulically actuatedcylinder connected to a ram for transferring molding material from achamber into a mold and including a source of pressurized hydraulicfluid having a pressure relief valve and connected directly to thecylinder, and a controller controlling operation of the molding press,the controller comprising(a) a controllable transducer operating thepressure relief valve to control hydraulic fluid pressure in thecylinder; (b) ram velocity control means comprising means sensing thevelocity of the ram as it moves in an injection stroke and meanscomparing the sensed velocity with a preset velocity and producing adeviation signal controlling the pressure relief valve transducer tothereby control the ram velocity; (c) holding pressure control meanscomprising sensing means sensing the hydraulic cylinder pressure, meanscomparing the sensed pressure with a preset value and producing adeviation signal controlling the pressure relif valve transducer tocontrol the hydraulic cylinder pressure; and (d) switching meansresponsive to the cylinder pressure sensing means switching control ofthe pressure relief valve transducer from the velocity control means tothe pressure control means when the cylinder pressure has attained apreset value.